Vacuum bulbs are used in electrical current-interrupter devices. A vacuum bulb is constituted by a cylinder of insulating material fitted with two facing electrical contacts. One of the contacts is stationary and the other is moving to enable the contacts to be separated or to be brought together. The contacts are connected to external conducting flanges placed on opposite faces of the bulb, which flanges enable electrical connections to be made with external lines or circuits.
The vacuum established inside the bulb provides the required dielectric strength and interrupting power in a small volume. However, it is necessary to provide a creepage distance outside the bulb of sufficient length to guarantee overall isolation between the contacts. Bulbs are then fitted with electrical insulators having fins in order to obtain creepage distances of sufficient length.
Such isolators are conventionally made of ceramics. An insulator is placed around a vacuum bulb between its flanges. The space between the bulb and the insulator is filled with a filler resin that must have very good adherence qualities and that must not shrink in order to avoid allowing a layer of air to form between the bulb and the resin or between the resin and the electrical insulator.
Ceramic insulators suffer from a certain number of drawbacks.
They are heavy, and above all they are brittle which makes them particularly sensitive to vandalism. When placed on the roof of a rail vehicle, their relatively large volume gives rise to drag.
As a replacement for ceramic material, it is known that a resin can be molded over a vacuum bulb to form an insulator. Appropriate resins include epoxies such as those sold under the trademark Araldite, after treatment to enable them to withstand ultraviolet radiation. However, such materials are also fragile, and in operation they require significant thicknesses, thereby giving rise to high weight.
Document WO 91/06106, filed in the name of Raychem Limited, discloses a high tension insulator having a polymer core which provides it with good mechanical strength and which is protected on the outside by a sheath of heat-shrink polymer which is electrically insulating and which withstands leakage currents, said sheath being provided with fins on its outside surface.
To mitigate the drawbacks of prior art vacuum bulbs provided with electrical insulation, the present invention seeks to dispose electrically insulating heat-shrink substances such as those disclosed in WO 91/06106 directly on a vacuum bulb. Such substances provide very good shock resistance because they are flexible and they enable insulators to be obtained that are compact and light in weight.